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Title: Near-wake dynamics of a vertical-axis turbine

Journal Article · · Journal of Fluid Mechanics

Cross-flow, or vertical-axis, turbines are a promising technology for capturing kinetic energy in wind or flowing water and their inherently unsteady fluid mechanics present unique opportunities for control optimization of individual rotors or arrays. To explore the potential for beneficial interactions between turbines in an array, as well as to characterize important cycle-to-cycle variations, coherent structures in the wake of a single two-bladed cross-flow turbine are examined using planar stereo particle image velocimetry in a water channel experiment. There are three main objectives in the present work. First, the mean wake structure of this high chord-to-radius ratio rotor is described, compared with previous studies, and a simple explanation for observed wake deflection is presented. Second, the unsteady flow is then analysed via the triple decomposition, with the periodic component extracted using a combination of traditional techniques and a novel implementation of the optimized dynamic mode decomposition. The latter method is shown to outperform conditional averaging and Fourier methods, as well as uncover frequencies suggesting a transition to bluff-body shedding in the far wake. Third, vorticity and finite-time Lyapunov exponents are then employed to further analyse the oscillatory wake component. Vortex streets on both sides of the wake are identified, and their formation mechanisms and effects on the mean flow are discussed. Strong axial (vertical) flow is observed in vortical structures shed on the retreating side of the rotor where the blades travel downstream. Time-resolved tracking of these vortices is performed, which demonstrates that vortex trajectories have significant rotation-to-rotation variation within one diameter downstream. This variability suggests it would be challenging to harness or avoid such structures at greater downstream distances.

Research Organization:
Maryland Energy Administration, Baltimore, MD (United States); Univ. of Washington, Seattle, WA (United States)
Sponsoring Organization:
USDOE Office of Energy Efficiency and Renewable Energy (EERE); Washington Clean Energy Fund; US Department of Defense (DoD); Army Research Office (ARO)
Grant/Contract Number:
EE006816; EE0008616; N00024-10-D-6318; 15-92201-006; W911NF-19-1-0045
OSTI ID:
1879561
Alternate ID(s):
OSTI ID: 1977725
Journal Information:
Journal of Fluid Mechanics, Journal Name: Journal of Fluid Mechanics Vol. 935; ISSN 0022-1120
Publisher:
Cambridge University Press (CUP)Copyright Statement
Country of Publication:
United Kingdom
Language:
English

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